Holographic recording for recording information on a recording medium through the use of holography is typically performed by superimposing light that carries image information on reference light within the recording medium and by writing a resultingly generated interference pattern onto the recording medium. For reproducing the information recorded, the recording medium is irradiated with reference light so that the image information is reproduced through diffraction derived from the interference pattern.
Recently, volume holography, or digital volume holography in particular, has been developed and is attracting attention in practical fields for ultra-high density optical recording. Volume holography is a method for writing a three-dimensional interference pattern by making positive use of a recording medium in the direction of thickness as well, and is characterized in that an increase in thickness can enhance the diffraction efficiency and a greater recording capacity can be achieved by employing multiplex recording. Digital volume holography is a computer-oriented holographic recording method which uses the same recording medium and recording method as with the volume holography, whereas the image information to be recorded is limited to binary digital patterns. In the digital volume holography, analog image information such as a picture is once digitized and developed into two-dimensional digital pattern information, and then it is recorded as image information. For reproduction, this digital pattern information is read and decoded to restore the original image information for display. Consequently, even if the SN ratio (signal-to-noise ratio) in the reproduction is somewhat poor, differential detection and/or error correction on encoded binary data enable to reproduce the original information with extremely high fidelity.
Now, in a conventional optical information recording/reproducing method for recording and reproducing information through the use of holography, information light is generated by spatially modulating the intensity of light based on the information to be recorded, and, an interference pattern resulting from interference between this information light and recording-specific reference light is recorded on a recording medium, to thereby record the information. To reproduce the information thus recorded, the recording medium is irradiated with reproduction-specific reference light. This reproduction-specific reference light is then diffracted by the interference pattern to generate reproduction light corresponding to the information light. Like the information light, this reproduction light is light that is spatially modulated in intensity.
By the way, the conventional optical information recording/reproducing method has had a problem that the reproduced information deteriorates in S/N ratio if the reproduction-specific reference light is also incident on a photodetector for detecting the reproduction light. For that reason, for the conventional optical information recording/reproducing method, the information light and the recording-specific reference light are in most cases allowed to be incident on the recording medium with a predetermined angle therebetween at the time of recording, so that the reproduction light and the reproduction-specific reference light can be spatially separated from each other at the time of reproduction. Consequently, the reproduction light, which occurs at the time of reproduction, travels at a predetermined angle with respect to the reproduction-specific reference light. This allows the reproduction light and the reproduction-specific reference light to be spatially separated from each other.
Nevertheless, when the information light and the recording-specific reference light are allowed to be incident on the recording medium with a predetermined angle therebetween at the time of recording so as to spatially separate the reproduction light and the reproduction-specific reference light from each other at the time of reproduction as described above, there arises a problem that the optical system for recording and reproduction becomes greater in size.